This application is related to SD-7221, xe2x80x9cMicrofabricated Teeter-Totter Resonator,xe2x80x9d filed of even date with this application.
The present invention relates to chemical sensing and, more particularly, to a method for chemical sensing using a microfabricated teeter-totter resonator.
Microfabricated chemical sensors based on acoustic devices combine a chemically sensitive interface, which sorbs chemical species (i.e., analytes) from the environment, with a physical transducer that provides an electrical output proportional to the amount of sorbed species. Two commonly used acoustic devices for chemical sensing include surface acoustic wave (SAW) and resonant microsensors, such as the flexural plate wave (FPW) resonator. See S. J. Martin et al., xe2x80x9cGas Sensing with Acoustic Devices,xe2x80x9d Proc. IEEE Ultrasonics Symposium, 423 (1996), which is incorporated herein by reference.
SAW sensors rely on the electrical excitation of a surface acoustic wave in a piezoelectric substrate. Typically, a wave is established on a quartz surface and the collection of analyte mass on the surface is reflected in the propagation of the surface wave. The high quality factor, Q, and low insertion loss of SAW chemical sensors makes them extremely stable in an oscillator circuit, resulting in low detection limits. SAW sensors can detect sub nanogram-levels of chemical analytes. However, SAW sensors typically operate at hundreds of megahertz frequencies, complicating the design and integration of oscillator circuitry.
A resonator comprises a vibrating element of a certain shape. Depending on the shape, the resonator can support several types of vibrations, e.g., longitudinal, transverse, torsional, and lateral, that can have a number of vibrational modes or resonances. The stress, mass, or shape of the resonator is typically designed such that one of these modes dominates and the resonant frequency of the dominant mode is matched to a driving excitation signal. When used as a chemical sensor, the collection of analyte mass or fluid loading on the surface of the resonator is reflected as a change in the resonant frequency or the amplitude of the vibration.
Resonators that operate on magnetic actuation principles are particularly attractive for chemical sensing applications, due to their large dynamic range and high sensitivity. Electromagnetic sensors rely on a Lorentz force, generated by an alternating electrical current flowing in the resonator interacting with an external magnetic field, to excite a mechanical vibration in the structure.
A magnetically excited FPW (mag-FPW) resonator is described in S. J. Martin et al., xe2x80x9cFlexural plate wave resonator excited with Lorentz forces,xe2x80x9d J. Appl. Phys. 83(9), 4589 (1998) and U.S. Pat. No. 5,836,203, which are incorporated herein by reference. The mag-FPW resonator comprises current lines patterned on a silicon nitride membrane that is suspended on a silicon frame. A Lorentz force is created by the interaction of an alternating surface current flowing in the current lines and an in-plane static magnetic field perpendicular to the current flow direction. Preferential coupling to a particular membrane mode is achieved by positioning the current lines along antinodes of the longitudinal mode. When the alternating current has the natural frequency of the mag-FPW resonator, a large amplitude standing wave is set up in the membrane wave plate. The motion of the current conductor lines in the magnetic field in turn induces a back electromotive force (back-emf) opposing the motion. This back-emf can be detected as an increase in impedance of the current lines.
Because the confinement of kinetic energy is in a thin, low-mass membrane, the FPW sensor can have a very high mass sensitivity. Also, because the wave velocity in the FPW membrane is much less than in a solid substrate, the operating frequency of a FPW device is much lower than in a SAW device, resulting in simpler oscillator electronics. In addition, FPW resonators can be made with micromachining processes in a silicon wafer and can be integrated with microelectronic circuits. However, the temperature-dependent tension variation in the membrane, due to the differential thermal expansion of the silicon nitride membrane relative to the silicon frame, make the FPW operation very sensitive to temperature drift.
Therefore, a need remains for a method for chemical sensing that uses a microfabricated resonator having a high Q-factor, low operating frequency, high mass sensitivity, and low temperature drift. The method of the present invention uses a microfabricated teeter-totter resonator that provides a Q-factor, operating frequency, and mass sensitivity comparable to the FPW resonators, but with much better temperature stability. In particular, the teeter-totter resonator is about three orders of magnitude less sensitive to temperature drift than the FPW resonator.
The present invention is directed to a method for sensing a chemical analyte in a fluid stream, the method comprising providing a microfabricated teeter-totter resonator, the resonator comprising a frame, a paddle having a first end and a second end and wherein the paddle is pivotably anchored to the frame by pivot arms at each end of the paddle, the pivot arms defining an axis of rotation of the paddle, a current conductor line on a surface of the paddle that is displaced from the axis of rotation of the paddle, means for applying a static magnetic field aligned substantially in-plane with the paddle and substantially perpendicular to the current conductor line and the axis of rotation, and means for energizing the current conductor line with an alternating electrical current; applying the static magnetic field; energizing the current conductor line with the alternating electrical current to excite an oscillatory motion of the paddle about the axis of rotation; exposing the resonator to the chemical analyte; and detecting the response of the oscillatory motion of the paddle to the chemical analyte. Preferably, a chemically sensitive coating is disposed on at least one surface of the paddle to enhance the sorption of the analyte from the fluid stream. The detecting can comprise measuring the change in resonant frequency or phase or rotational displacement of the oscillatory motion of the paddle due to fluid loading or sorption of the analyte by the paddle.